1. Field of the Invention
The present invention relates to an image pickup lens system and particularly, to an image pickup lens system which is comprised of three lenses and used in an image pick-up device (e.g., an image-inputting CCD camera) utilizing a solid image-pickup element such as CCD, CMOS and the like mounted on a potable computer, a visual telephone and the like, and which can be reduced in size and weight.
2. Description of the Related Art
In recent years, multimedia have been developed remarkably, and for example, the demand for a camera utilizing a solid image-pickup element such as CCD, CMOS and like, e.g., a CCD camera, mounted on a portable computer, a visual telephone and the like, is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight. Therefore, it is also desired that an image pickup lens system used in such a CCD camera is likewise small in size and lightweight.
A one-lens system using a single lens or a two-lens system using two lenses is conventionally used as such an image pickup lens system.
However, such a lens system is convenient extremely for reducing the size and weight of the lens, but recently suffers from a problem that it is not suitable for increasing the picture quality and the resolution required for the image pick-up lens system.
For this reason, it is a conventional practice to use a three-lens system using three lenses, thereby providing increases in picture quality and in resolution.
Such a three-lens system has a long history in the field of a silver-salt photograph camera, and optical lens systems of various arrangements have been developed.
In the lens system in the silver-salt photograph camera, however, if the lens is used in a configuration as it is, the lens is difficult to produce, because of the relationship between the lens thickness and the focal length and the relationship between the lens thickness and the lens diameter. For this reason, it is impossible to utilize the lens as an image pickup lens for an image pickup element.
Therefore, a three-lens image pickup lens system exclusively used in an image pickup element has been conventionally developed. One example of such image pickup lens system is a lens system comprising a lens having a negative power, a lens having a negative power and a lens having a positive power, which are arranged sequentially from the side of the object.
However, the conventional image pickup lens system of such arrangement suffers from the following problems: Various aberrations including mainly a chromic aberration cannot be corrected appropriately, and it is difficult to ensure a large distance from an image surface to an exit pupil.
To solve such problems, it is effective to provide a diaphragm at a location closer to an object from an optical system (a group of lenses). Such image pickup lens systems include those disclosed, for example, in Japanese Patent Application Laid-open Nos. 4-153612, 5-188284 and 9-288235.
Any of these image pickup lens systems disclosed in the above Patent Applications is comprised of a diaphragm, a first lens, a second lens and a third lens, which are disposed sequentially from the side of an object. The first lens is a lens which has a first face formed as a convex face and has a positive power; the second lens is a lens which has a first face formed as a concave face and has a negative power; and the third lens is a lens having a positive power.
However, any of such conventional image pickup lens systems suffers from a problem that a larger angle of view of the image pickup lens cannot be ensured, and the maximum limit of angle of view capable of being ensured is about 50xc2x0, because the first face of the first lens is formed into the convex face.
Any of the conventional image pickup lens systems disclosed in the above Patent Applications suffers from a problem that a special glass material having a high refractive index on the order of 1.7 or a super-high refractive index of 1.8 or more is used for producing the lens and for this reason, a reduction in weight of the lens system cannot be achieved and moreover, the manufacture cost is increased. A further problem encountered in each of the conventional image pickup lens systems disclosed in the above Patent Applications is that when an inexpensive glass or resin material such as a plastic is used to provide reductions in weight and cost, it is impossible to ensure a desired optical performance of each of the lenses.
Accordingly, it is an object of the present invention to provide an image pickup lens system, in which an inexpensive glass or plastic material is used for producing each of lenses, but still, various aberrations can be corrected satisfactorily, while maintaining a desired optical performance and ensuring a wider angle of view, and which can be produced easily.
To achieve the above object, according to the present invention, there is provided an image pickup lens system comprising a diaphragm, a first lens which has a convex face formed on the side of an image surface in the vicinity of an optical axis and which has a positive power and radii of curvature at a center, whose signs are not opposite from each other, a second lens which has a convex face formed on the side of an object in the vicinity of the optical axis and which has a negative power and radii of curvature at a center, whose signs are not opposite from each other, and a third lens which has a convex face formed on the side of the image surface in the vicinity of the optical axis and which has a positive power, said diaphragm and said first, second and third lenses being disposed sequentially from the side of the object, the first face of at least said second lens being formed into an aspherical shape.
With the above feature, the second face of the first lens adjacent the image surface is the convex face, and the first face of the fist lens adjacent the object is not opposite in sign from the second face and hence, the first face of the first lens may be a concave or flat face. Thus, a wider angle of view can be ensured.
According to the present invention, the first lens satisfies the following expression:
r1xe2x89xa61.5xc3x97r2
wherein r1 is a radius of curvature at the center of the first face of the first lens adjacent the object, and r2 is a radius of curvature at the center of the second face of the first lens adjacent the image surface.
With the above feature, the above expression is a condition for ensuring that various aberrations can be corrected satisfactorily, while ensuring a desired optical performance and maintaining a wider angle of view, and the image pickup lens system can be produced easily. By setting the value of r1 in the expression at a value equal to or smaller than 1.5xc3x97r2, each of various aberrations, particularly, a distortion, can be suppressed to an acceptable level, while ensuring a wider angle of view. Further, by setting the value of r1 in the expression at a value equal to or smaller than 1.7xc3x97r2, the generation of the aberrations can be inhibited.
According to the present invention, the second lens satisfies the following expression:
0.45xc3x97r3xe2x89xa7r4
wherein r3 is a radius of curvature at the center of the first face of the second lens adjacent the object, and r4 is a radius of curvature at the center of the second face of the second lens adjacent the image surface.
With the above feature, the above expression is a condition for ensuring that each of various aberrations can be corrected satisfactorily, while maintaining a desired optical performance and a wider angle of view, and the image pickup lens system can be produced easily. By setting the value of r4 in the expression at a value equal to or smaller than 0.45xc3x97r3, each of the aberrations such as a magnification chromic aberration can be corrected satisfactorily.